Half tone development process for touchdown system in electrostatic imaging

ABSTRACT

A touchdown system for obtaining many distinctive steps in grey scale rendition when developing solid areas comprises a toner laden donor member and a toner layer pattern forming member. The pattern forming member has an electrode arranged in such a manner thereon that when a predetermined field is established between the electrode and the donor member, a predetermined pattern of peaks and valleys will be formed in the toner layer. When this patterned toner layer is presented to an electrostatic solid area latent image, the weakest charged portions of the latent image will attract only the toner from the portion of the layer which is closest thereto i. e., the peaks, thereby presenting a dot pattern development thereon. The strongest charged portion of the electrostatic latent image will attract toner not only from the closet portion or peaks but will also attact toner from the valleys which will fully develop the image where the charge is strongest. The charged pattern of the latent image which is between the weakest and strongest charge will attact a broadening pattern of toner from the peaks as well as from the valleys as the charge gets stronger.

United States Patent 1 Fantuzzo May 6, 1975 HALF TONE DEVELOPMENT PROCESS FOR TOUCHDOWN SYSTEM IN ELECTROSTATIC IMAGING [21] Appl. No.: 351,221

[52] US. Cl 96/15 D; 117/175; 118/637; 355/3 DD [51] Int. Cl G03g 13/08 [58] Field of Search 96/15 D; 117/175; 118/637; 335/3 DD [56] References Cited UNITED STATES PATENTS 3,011,473 12/1961 Gundlach 118/637 3,152,012 10/1964 Schaffert....... 113/637 3,203,394 8/1965 Hope et a1. 117/17.5 X 3,559,570 2/1971 Martel et a1. 118/637 X 3,635,195 1/1972 Levy n 118/637 3,696,783 10/1972 Fantuzzo 118/637 3,696,785 10/1972 Andrus 118/637 3,731,146 5/1973 Bettiga et a1 118/637 X 3,739,748 6/1973 Rittler et a1 118/637 3,754,962 8/1973 Berber et a1. 96/15 D X 3,754,963 8/1973 Chang 118/637 X 3,759,222 9/1973 Maksymiak et a1 96/15 D X VACUUM VACUUM opment, IBM Technical Disclosure Bulletin, Vol. 12, No. 12, May, 1970, page 2280.

Primary ExaminerNorman G. Torchin Assistant Examiner-John R. Miller [5 7] ABSTRACT A touchdown system for obtaining many distinctive steps in grey scale rendition when developing solid areas comprises a toner laden donor member and a toner layer pattern forming member. The pattern forming member has an electrode arranged in such a manner thereon that when a predetermined field is established between the electrode and the donor mem her, a predetermined pattern of peaks and valleys will be formed in the toner layer. When this patterned toner layer is presented to an electrostatic solid area latent image, the weakest charged portions of the latent image will attract only the toner from the portion of the layer which is closest thereto i. e., the peaks, thereby presenting a dot pattern development thereon. The strongest charged portion of the electrostatic latent image will attract toner not only from the closet portion or peaks but will also attact toner from the valleys which will fully develop the image where the charge is strongest. The charged pattern of the latent image which is between the weakest and strongest charge will attact a broadening pattern of toner from the peaks as well as from the valleys as the charge gets stronger.

3 Claims, 4 Drawing Figures HALF TONE DEVELOPMENT PROCESS FOR TOUCHDOWN SYSTEM IN ELECTROSTATIC IMAGING This invention is concerned with an improvement to a touchdown development system. The term touchdown development as used in this application refers to the following system:

A toner laden rotating donor member (which may be a roller or an endless belt) is located adjacent to a moving photoreceptive surface bearing a latent electrostatic image thereon. The donor member is spaced from the photoreceptor so that a gap of about 2 to E microns exists between the toner layer surface and the photoreceptive surface. The toner on the donor member carries a charge in polarity opposite to the polarity of the electrostatic latent image. As the donor member carries toner past the latent image, the toner is attracted from the donor to the image to develop the image.

In developing solid areas with this touchdown system, there have been problems with achieving more than about three distinctive steps of grey scale rendition.

Accordingly. it is an object of this invention to provide a touchdown development system which provides for half-tone development of solid areas resulting in many distinctive steps of grey scale rendition.

Other objects of this invention will become apparent from the following description with reference to the drawings wherein:

FIG. 1 is a schematic view of a touchdown development system;

FIG. 2 is a partial schematic view of a microfield donor and a commutating system therefor;

FIG. 3 is a view of a tonor layer pattern forming roll; and

FIG. 4 is a view of the donor roll with a toner layer thereon shown in section after a pattern has been formed therein.

Referring to FIG. I, there is shown a xerographic reproduction system utilizing the concept of the present invention. In this apparatus a xerographic plate is in the form of a drum l0 which passes through stations A through E in the direction shown by the arrow. The drum has a suitable photosensitive surface on which a latent electrostatic image can be formed. The plurality of stations about the periphery of the drum which carry out the reproduction process are: charging station A. exposing station B, developing station C, transfer station D, and cleaning station E. Stations A, B, D, and E represent conventional means for carrying out their respective functions and do not form a part of the present invention.

At station A. a corona charging means 12 places a uniform electrostatic charge on the photoconductive material. As the drum rotates. a light pattern, by a suitable exposing apparatus I4, is exposed onto the charged surface of drum I0. The latent image thereby formed on the surface of the drum is developed by the application of toner particles at developing station C, which is described in greater detail below. After the image is developed at station C, it passes through transfer station D. wherein the toner is transferred to a sheet I6. A corona charging device 18 is utilized in the normal way to facilitate transfer and fuser device 20 is provided to fuse the toner to sheet I6. Following transfer of the developed image to the copy sheet. the drum ro- 2 tates through cleaning station E, comprising cleaning brush 22 which cleans residual toner particles from the drum 10.

Referring to FIG. 2, the apparatus includes a donor member 24 at the developing station C which comprises an aluminum drum 26 and a dielectric layer 28 separating the drum 26 from a copper grid pattern 30. There are a plurality of electrically isolated grid patterns 30 to allow a varying potential to be applied thereto in accordance with a position of a particular portion of the donor member 24 with respect to the various stations located around it. This is accomplished by connecting the aluminum drum to ground by contacting a brush 34 with the inner surface thereof and with an electrical lead 36. Individual electrical connections are made from each grid pattern 30 to a programmed means which will permit each grid pattern to be placed at ground potential or to some desired charge potential. This programmed means comprises a segmented slip ring 37 and electrical contacts 38 which are shown schematically in FIG. 2. In actual practice, a respective electrical lead 39 could be located inside of the donor member in electrical communication with a respective grid pattern while the other end of the respective electrical lead 39 is connected to a respective contact 38 which is in ring 37. A voltage source 42 can supply the appropriate potential to each segment 40 of the slip ring.

The donor member 24 is rotatably mounted adjacent a toner reservoir 44 containing a supply of toner particles 46 in order that a portion of its periphery comes into contact with toner 46. This portion of the donor member 24 is properly biased to attract a layer 47 of toner particles thereto. The donor roll is also located so as to provide a small gap of approximately 2-10 microns between the surface of drum l0 and the outer surface of a toner layer carried by donor roll 24. After a particular portion of the donor roll is rotated through the toner reservoir. that portion of the donor rotates past a corona charging device 48 which charges the toner particles 46 to the correct polarity. Thereafter the charged toner layer 47 is rotated past a toner layer pattern forming a gravure roll 50, whereby a peak and valley pattern is formed in the toner layer 47, and then the toner layer 47 is presented to the latent image to develop the same.

Following development, the donor roll is prepared for toner reloading by exposing the residual toner thereon to a neutralizing corona charge means 52 to make easier the removal of the residual toner by way of a cleaning brush 54 equipped with a vacuum means 56.

Referring to FIG. 3, the gravure roll comprises a support drum 62 on which a copper film electrode 64 in the form of a grid pattern is fixed. The support may be either a conductive or a dielectric material. The roll 50 is so positioned that outer surface of the grid pattern 54 is spaced 2 to ID microns from the outer surface of the toner layer on the donor roll 24. A voltage potential V is applied to the grid 64. That portion of the donor opposite the gravure roll 50 is programmed to be at ground potential. The attractive field set up by the potential between the donor roll 24 and the grid 64 is pre determined so that a substantial amount of toner is removed from the toner layer and attracted to the grid 64 thereby leaving a predetermined pattern of individual peaks 66 surrounded by a continuous valley 68 in the toner particle layer as shown in FIG. 4. The valleys 68 correspond to the toner removed by the grid electrode 64.

When the donor 24 presents the patterned toner layer 47 to the electrostatic latent image, the weakest charged portion of the image will attract only the toner from the portion of the layer which is closest thereto, i.e., the peaks 66 thereby presenting a dot pattern development thereon. The strongest charged portion of the electrostatic latent image will attract toner not only from the closest portion or peaks 66 but will also attract toner from the valleys 68 which will fully develop the image where the charge is strongest. The charged pattern of the latent image which is between the weakest and strongest charge will attract a broadening pattern of toner from the peaks 66 as well as from the valleys 68 as the charge gets stronger. Thus, one can see that there will be many distinctive steps in grey scale rendition.

The developed image characteristics can be varied in accordance with the attractive field set up by the potential between the donor 24 and the gravure grid 64 which can be in a range between the electrostatic latent image background potential and the full potential on the photoreceptor. Thus, assuming a positive charge on the photoreceptor and negative charged toner, the potential on the grid 64 can be in a range between +100 volts to +800 volts while the potential of the donor can be ground potential. As the lower potential range on the grid 64 is approached, less toner will be removed from the toner layer resulting in shallow and narrow valleys, while as the upper potential range on the grid 64 is approached, more toner will be removed from the toner layer resulting in deeper and wider valleys. For all practical purposes, when the potential of the gravure grid 64 is at background potential, the toner layer on the donor roll 24 is such that normal development takes place rather than half-tone development and therefore this potential on the gravure roll could be utilized when half-tone development is not desired. At this background potential, the gravure roll will act as a roll means for removing a substantial amount of toner from the layer 47 which would otherwise be attracted to the background. When the gravure grid 64 is at a potential which is the full potential on the photoreceptor, only half-tone development will occur on the highest charged portions of the image. Thus, if only half-tone development without much distinction in grey scale rendition is desired, the gravure grid 64 would be set at full photoreceptor potential.

From the above, it can be seen that when a given electrode pattern is utilized for the gravure roll, the distinctive steps in grey scale rendition obtained by the method of this invention can be changed by varying the potential on the gravure roll. Also, by changing the electrode pattern on the gravure roll, the distinctive steps of grey scale rendition may also be varied. The electrode may be any type of pattern desired as long as it will form a pattern of peaks and valleys in the toner layer 47 on the donor roll 24.

While the donor roll 24 is described as a microfield donor roll, any type donor roll may be utilized. The key factor is that the donor roll must have a layer of toner thereon and that a field be set up between the donor roll and the gravure roll which will attract the particles to the gravure roll in such a manner that peaks and val leys are formed in the toner layer.

While the photoreceptor 10, the donor roll 24, and the pattern-forming member 50 have each been described as rolls, any one of them or all of them may be in the form of a belt.

What is claimed is:

l. A method for developing latent electrostatic images carried by a substrate; said method comprising the steps of loading a donor member with a layer of toner,

imparting a uniform electrical charge of predetermined polarity to said donor layer,

subjecting the charged toner to an electrostatic field having an intensity varying in accordance with a predetermined grid-like pattern to create a corre sponding pattern of hills and valleys in said layer of toner, and

presenting the patterned toner layer of said doner member to said substrate for developing said images, whereby any gray scale content of said images tends to be preserved.

2. The method of claim 1 further including the step of rotating said donor member to carry out said loading, charging, pattern creating and developing steps in sequence.

3. The method of claim I further including a postdevelopment step of cleaning residual toner from said donor member in preparation for a subsequent cycle. k k 

1. A METHOD FOR DEVELOPING FOR DEVELOPING LATENT ELECTROSTATIC I CARRIED BY A SUBSTRATE, SAID METHOD COMPRISING THE STEPS OF LOADING A DONOR MEMBER WITH A LAYER OF TONER, IMPARTING A UNIFORM ELECTRICAL CHARGE OF PREDETERMINED POLARITY TO SAID DONOR LAYER, SUBJECTING THE CHARGED TONER TO AN ELECTROSATIC FIELD HAVING AN INTENSITY VARYING IN ACCORDANCE WITH A PEDETERMINED GRID-LIKE PATTERN TO CREATE A CORREPSONDING PATTERN OF NILLS AND VALLEYS IN SAID LAYER OF TONER, AND PRESENTING THE PATTERNED TONER LAYER OF SAID DONER MEMBER TO SAID SUBSTRATE FOR DEVELOPING SAID IMAGES, WHEREBY ANY GRAY SCALE CONTENT OF SAID IMAGES TENDS TO BE PRESERVED.
 2. The method of claim 1 further including the step of rotating said donor member to carry out said loading, charging, pattern creating and developing steps in sequence.
 3. The method of claim 1 further including a postdevelopment step of cleaning residual toner from said donor member in preparation for a subsequent cycle. 